1. Field of the Invention
The present invention relates to a data storage apparatus for recording and reproducing information to a disk-shaped recording medium such as an optical or magnetic disk.
2. Description of the Prior Art
Advances in semiconductor technologies in recent years have significantly improved digital signal processing capabilities, making it possible even for personal computers and other low-cost electronics to process large volumes of digital data at high speed. As processing capabilities have developed, demand has also grown for read/write data storage media enabling even larger amounts of data to be accessed at high speed. The most common methods today record and reproduce data to/from a disk-shaped medium (referred to simply as "disk" below) using a read/write head. Common examples of such disks today include magnetic hard disks and optical disks.
The disk is driven by a motor or other rotational drive means at a rotational velocity (revolutions per unit time) V.sub.A relative to the optical head. The laser spot emitted from the optical head is focused on the recording thin-film layer. Any of various known focus control technologies may be used to focus the laser beam to have a spot on the recording thin-film layer. The temperature of the recording thin-film layer increases through laser absorption. When the laser spot output exceeds some variable threshold value dependent upon the data to be recorded, a local state change occurs in the recording thin-film layer, and data is recorded by controlling this state change. This threshold value is a quantity that is dependent on the specific characteristics of the recording thin-film, the thermal characteristics of the substrate, the linear velocity V.sub.L of the disk to the optical head and other factors. The recording state varies according to the size of the laser beam, the laser output power or pulse is width, and other recording conditions.
In FIG. 8, the relationship between recording position and both linear velocity V.sub.L and rotational velocity V.sub.A in a conventional recording and reproducing method which is commonly called the constant angular velocity (CAV) method is shown. In CAV method, the disk 1 is typically driven at a constant speed V.sub.A while the data is recorded or reproduced from the disk 1, wherein the disk 1 has sectors comprised of a plurality of tracks divided by a predetermined angular. Since the disk 1 is rotated at a predetermined rotational velocity V.sub.A, the linear velocity V.sub.L of the recording head to the disk track is determined by the radial position of the recorded track, being proportional to the radius such that the linear velocity V.sub.L increases toward the outside circumference of the disk and decreases toward the inside circumference.
In FIG. 10, a graph showing an example of the relationship between recording position and both linear velocity V.sub.L and rotational velocity V.sub.A according to CAV method is shown. When a 130 mm diameter disk is driven at a rotational velocity V.sub.A of 1800 rpm with data recorded to the area between a 30 mm and 60 mm radius R, the linear velocity V.sub.L under these conditions ranges from 5.65 m/sec at the smallest inside circumference to 11.30 m/sec at the greatest outside circumference, yielding a maximum to minimum ratio of 2:1.
In FIG. 9, the relationship between recording position and both linear velocity V.sub.L and rotational velocity V.sub.A in another conventional recording and reproducing method which is called the constant linear velocity (CLV) method is shown. In the CLV method, the rotational velocity V.sub.A of the disk 1 is varied to maintain a constant linear velocity V.sub.L from the inside R.sub.O to outside Rn circumferences of the disk 1. The disk 1 has sectors comprised of a plurality of tracks having a predetermined length. To record and reproduce the information at a predetermined linear velocity V.sub.L, the rotational velocity V.sub.A of the disk 1 shall be reduced to from the inside to outside circumferences of the disk 1, as shown in FIG. 8.
In FIG. 11, a graph showing an example of the relationship between recording position and both linear velocity V.sub.L and rotational velocity V.sub.A according to CLV method is shown. When a 130 mm diameter disk is driven to maintain a constant linear velocity V.sub.L of 5.65 m/sec, data is recorded to the area between a 30 mm and 60 mm radius R. The rotational velocity V.sub.A of the disk 1 is therefore varied continuously from a maximum 1800 rpm at the inside circumference to a minimum 900 rpm at the outside circumference of the disk 1.
In both the CLV and CAV methods, however, a constant relationship between radius R and rotational velocity V.sub.A or linear velocity V.sub.L is maintained in both data recording and reproducing operations.
The problem with the CAV method, however,. is that the linear velocity V.sub.L differs at inside and outside disk circumferences. The optimum conditions for recording therefore change as the linear velocity V.sub.L changes, and the recording conditions of the recording head must be continuously modified according to the radial position of the recording head to the disk 1.
While the use of a constant linear velocity V.sub.L in the CLV method means it is not necessary to modify the recording conditions of the recording head, it is necessary to change the rotational velocity V.sub.A of the disk according to the radial position of the recording track. This is particularly problematic when randomly accessing any given track because the time (drop time) required to change the rotational speed of the motor or other drive means to the value required by the radial position of the track increases the access time.
When the linear velocity V.sub.L changes, the threshold value of the laser power required to record data also usually changes with a relatively higher power level required as the linear velocity V.sub.L increases. When the power level is too high, however, the recording medium can be damaged. It is therefore necessary to optimize the recording power according to the linear velocity V.sub.L.
When using a phase-change optical recording medium, to which data is recorded by inducing a phase change (e.g., the crystal state) in the recording thin-film, the laser spot is emitted at a relatively high recording power level to melt and rapidly cool the recording thin-film. This induces an amorphous state in the crystal thin-film to enable data recording. A relatively low erase power level is used to crystallize the thin-film in a solid state and thereby erase data. Crystallization requires a certain amount of time because the atoms are rearranged. When the linear velocity V.sub.L is high and a phase-change recording medium is used, the high disk speed results in a smaller increase in thin-film temperature per unit time because the thin-film is exposed to the laser spot for less time. This results in insufficient crystallization and a drop in erase performance. When the linear velocity V.sub.L is low, the thin-film melts due to exposure to the laser beam at the recording power level, but the normal rapid cooling conditions are not obtained the thin-film therefore cools slowly, tending to crystallize during the solidification process and inhibiting the normal formation of amorphous recording marks. The general problem with this method can therefore be summarized as the recording state of the optical disk being significantly dependent upon the relative linear velocity V.sub.L of the optical head to the recording track.